1. Field of the Invention
The present invention relates to a drive control for a motor, and particularly relates to a drive control for a motor such as a stepping motor that can be used as a driving source for controlling conveyance of a recording material in an image forming apparatus such as a copying machine or a printer.
2. Description of the Related Art
In electrophotographic image forming apparatuses such as copying machines, printers, and the like, stepping motors are widely used as a driving source of a conveyance system for conveying a recording material such as a sheet on which an image is formed, and the like. A stepping motor has advantages that even if a mechanism for detecting a position and rotation speed of the motor is not provided, it is possible to perform speed control easily by controlling a pulse period applied to the motor, and also it is possible to perform position control easily by controlling the number of pulses applied to the motor. (This method of control is referred to generally as synchronization control.) However, when performing synchronization control of a stepping motor, if a load torque necessary in the apparatus exceeds a torque range that can be output, a step-out state which is a state in which synchronization with an input pulse is not performed and it is uncontrollable is entered. If the motor enters a step-out state, conveyance of recording materials becomes impossible. This results in the occurrence of a paper jam, and it becomes necessary for the user to remove the paper stuck in the image forming apparatus. To avoid this, a motor output torque capable of supporting a change of the load torque that becomes necessary in the apparatus is required. For this, it is necessary to supply a driving current to the motor in order for a motor output torque having a predetermined margin with respect to the load torque necessary in the apparatus to be obtained. There was a problem in that as a result of this, more power was consumed than necessary, and vibrations and noise would occur in the apparatus due to surplus torque.
As a technique for dealing with such problems, a method called vector control (also FOC: Field Oriented Control) has been proposed as recited in U.S. Pat. No. 6,850,027 and U.S. Publication No. 2011/0285332. Vector control is a method of controlling amplitude and phase of a driving current so that a maximum torque is generated for a motor using a rotating coordinate system defining a rotor magnetic flux direction as a d axis, and a direction orthogonal thereto as a q axis. In the rotating coordinate system, a q axis component (a q axis current) of the driving current is a torque current component that generates torque, and a d axis component (a d axis current) of the driving current is an excitation current component that influences the rotor magnetic flux intensity. In particular, torque control is possible using only the q axis current without requiring a d axis current in a motor that uses a permanent magnet for a rotor as with a stepping motor. As a result, because the driving current of the motor in the stationary coordinate system becomes an ideal sinusoidal wave, not only is it possible to realize a drive control for which power efficiency is good, vibration and noise of the apparatus caused by surplus torque as described above are suppressed.
In the foregoing vector control, it is necessary to detect the position and the rotation speed of the rotor, and it is typical to perform the detection using a rotary encoder. However, there are problems of a cost increase and layout space allocation in the case where vector control is realized using a rotary encoder which is unnecessary in a conventional stepping motor control. For this reason, it is desired to be able to realize vector control without using a rotary encoder. In U.S. Publication No. 2011/0285332, a method of estimating position and rotation speed of a rotor without using a rotary encoder has been proposed. Specifically, by detecting a motor driving current, and calculating an arctangent of an induced voltage ratio in an A-phase and a B-phase estimated based on a voltage equation, the position of the rotor is estimated. A rotation speed of the rotor is estimated by a temporal differentiation of the estimation result for the rotor position.
If the position of the rotor is estimated at a certain control period using the method proposed in U.S. Publication No. 2011/0285332, an error occurs in the estimated position of the rotor due to a delay accompanying the estimation computation for estimating the position. In particular, the estimated position error becomes larger in a region where the number of rotations of the rotor is higher. In U.S. Publication No. 2011/0285332, the influence of this kind of estimated position error on the vector control is not made clear. However, according to the simulations and tests by the inventors, the estimated position error of the rotor was found to actually not only worsen the motor control efficiency significantly in the vector control, but also to bring the motor to an uncontrollable state.